1. Field of the Invention
This invention relates to an oxygen sensor for measuring oxygen concentration in a gas such as an exhaust gas, and more particularly to an oxygen sensor which can accurately determine air-fuel ratio or the like of a gas being measured with a comparatively low dependence on temperature.
2. Description of the Prior Art
To improve fuel cost and cleanness of exhaust gas of an automobile engine, it has been proposed to run the engine with an intake air-fuel mixture of lean burn side, i.e., with an air-fuel ratio .lambda., or an excess air ratio, larger than the theoretical optimal value at unity (.lambda.=1). To this end, there is a need for an oxygen sensor which can accurately measure the air-fuel ratio of unity or larger than unity .lambda..gtoreq.1. One example of such oxygen sensors of the prior art was disclosed in Japanese Patent Laying-open Publication No. 130,649/81 which was filed by the Ford Motor Company of the U.S.A.
The above-mentioned Ford's oxygen sensor uses two sintered plates of oxygen-ion-conductive solid electrolyte each of which has electrodes attached to opposite surfaces thereof. One of the sintered plates is used as an oxygen pump element while the other one of them is used as an oxygen concentration cell element. The oxygen pump element and the oxygen concentration cell element are attached to opposite surfaces of a cylindrical spacer so as to sandwich the sidewall of the cylindrical spacer by the two elements. The sidewall of the cylindrical spacer is made of a refractory material and has fine holes bored therein, so that an enclosed space is defined between the above-mentioned two elements while oxygen-diffusing holes are defined by said fine holes of the sidewall of the cylindrical spacer. The oxygen concentration of a gas can be electrically measured by placing the oxygen sensor in the gas, and applying an electric current through the oxygen pump element so as to pump out the oxygen from the above-mentioned enclosed space to the outside atmosphere or the gas being measured, while allowing diffusion of oxygen into the enclosed space through the oxygen-diffusing holes of the cylindrical spacer sidewall, until for instance an oxygen concentration ratio between the enclosed space and the outside atmosphere of the gas being measured reaches a certain stable value. The last mentioned oxygen concentration ratio is given by the oxygen concentration cell element as an output thereof, and the magnitude of the current applied to the oxygen pump element for pumping out oxygen corresponds to the oxygen concentration in the outside atmosphere of the gas being measured. This oxygen sensor uses the oxygen pump element and the oxygen concentration cell element which are separately formed, so that the output from the oxygen sensor has an advantage in that the dependence of the output thereof on the temperature of the outside atmosphere of the gas being measured, namely the temperature of the oxygen sensor, is low. However, the above oxygen sensor of the prior art has shortcomings in that its response characteristics are not good because the oxygen-diffusing holes of the cylindrical spacer sidewall carrying the two elements are comparatively very small relative to the volume of the enclosed space, and that the oxygen diffusing-holes are apt to be contracted or plugged by deposits from the gas being measured such as automobile engine exhaust gas. Accordingly, accurate measurement of the oxygen concentration was often impossible due to such concentration or plugging of the oxygen-diffusing holes. Even if the oxygen-diffusing holes are not plugged, slight deposits have proved to seriously affect the efficiency of the oxygen diffusion into the enclosed space because the diameter of such oxygen-diffusing holes is very small. Thus, the accuracy in the oxygen concentration measurement is easily deteriorated by the deposits.